The present invention relates to remote node synchronization, and more particularly, to remote node synchronization over a packet-switched network where all intermediary nodes are not necessarily synchronized.
Because synchronous networks tend to be expensive, packet-switched networks are usually not synchronized meaning that there is no common reference clock in the network. The IEEE 802.3 standard, often referred to as Ethernet, is an example asynchronous network that uses free-running clocks in all nodes. Although asynchronous networks are appropriate for many applications, in other applications, synchronization is important or otherwise desirable.
One example application where synchronization is important is radio access network (RANs) used in mobile radio communications. Present day, circuit-switched RANs use plesiochronous digital hierarchy (PDH), (e.g. E1), or synchronous digital hierarchy (SDH), (e.g., STM-1), links between RAN nodes. Due to the well-controlled jitter and wander characteristics of these transmission technologies, clock-recovery techniques can be utilized to reach 50 parts-per-billion (ppb) frequency accuracy necessary for the third generation, wideband code-division multiple access (WCDMA) air interface.
Instead of using circuit-switched communications where synchronization is based on SDH or PDH, it would be desirable to employ packet-switched network technologies for RAN node communications since packet-switched network infrastructures already exist. If that were to occur, there will be intermediary repeaters, switches, and routers (hereafter referred to as intermediary nodes) between the RAN nodes like radio base stations (RBSs), radio network controllers (RNCs), or the like. The intermediary nodes inject delays and uncertainties into the synchronization process not encountered in circuit-switched networks. Nonetheless, certain applications, such as WCDMA as mentioned above, require frequency-synchronization at a very high accuracy, and may also require absolute time synchronization. The latter is important for several positioning method of cellular terminals, e.g., Global Positioning System (GPS)-assisted positioning. Moreover, clock-recovery techniques can not be used for packet-switched, asynchronous network technologies like Ethernet, Internet Protocol (IP), or Asynchronous Transfer Mode (ATM).
A solution is needed that generally provides means for remote node synchronization over packet-switched networks between a sending node and a receiving node without requiring synchronization of any intermediary nodes, That solution should also solve specific problems and needs associated with synchronizing RAN nodes that employ packet-switched communications, e.g., very accurate synchronization for both frequency and absolute time.
Synchronization over packet-switched networks can be achieved using several approaches. One approach is to adjust the receiving node's clock based on the “filling level” of an elastic jitter buffer, as shown in FIG. 1. Upper and lower window boundaries are defined around the middle of the jitter buffer. For every n samples, the average position of a buffer pointer is calculated. In normal operation, the average pointer position should be around the middle of the window. If the average pointer position goes above the upper window boundary or below the lower window boundary, the receiving node's clock is corrected to return the average pointer position to the middle of the buffer.
Drawbacks of this approach include sensitivity to frame/packet loss. If a frame/packet is lost, the buffer decreases. Another drawback is the requirement for timeservers with very accurate periodicity. Few accurate periodicity timeservers which allow ppb accuracy to be reached are available on the market, and they are slow, with a periodicity on the order several seconds.
Another approach for frequency synchronization over packet-switched networks employs periodic timestamp transmission illustrated in FIG. 2. A Timeserver Q (e.g., an RNC) sends timestamps to a client receiver P (e.g., an RBS) with a predetermined periodicity. The frequency drift and time offset between Q and P are estimated from the timestamp periodicity so the periodicity must be accurate. Drawbacks of this approach are similar to those described for the jitter buffer approach described above including sensitivity to frame/packet loss and timeservers with very accurate periodicity.
A third approach relies on time differences between the timeserver Q and the client P. Advantages include insensitivity to frame/packet loss and no periodicity requirement. FIG. 3 illustrates a one-way timestamp procedure. A timestamp message, e.g., a network time protocol (NTP) message, is sent from the timeserver to the client through a number of intermediary nodes (e.g., switches). When sending the message, the timeserver inserts an absolute local time t3 in the message. When the client receives the message, it adds the absolute local time t4 to the message. The differential time Δt43=t4−t3 can then be calculated and evaluated by the client.
The differential time, in this case Δt43=t4−t3, is compared with the absolute local time t4 in the client. FIG. 4 shows multiple differential times Δt43 being plotted relative to the absolute local time t4. An oscillator frequency drift in the client appears as a drift in the differential time which is shown in FIG. 4 as a dashed line. The drift slope ρ (i.e., the slope of the dashed line), corresponds to the client's frequency drift compared to the timeserver. A least-squares algorithm may be used to estimate the differential time drift, but it unfortunately requires a long convergence time.